Formed-in-place membranes have many attractive features in separations by filtration, whether the application is microfiltration (of particulates), ultrafiltration, or hyperfiltration (reverse osmosis). The variety of materials from which they can be formed allows wide options in meeting severe requirements posed by temperature and aggressive chemical nature of feeds. An important characteristic is that when performance of the membranes deteriorates from fouling or other causes, they can be stripped and replaced in situ.
For certain uses, present formed-in-place membranes have limitations, as membranes based on hydrous Zr(IV) oxide illustrate. The removal and replacement of these membranes requires several hours. This is of no concern if membrane performance is satisfactory over weeks, but in cases where replacement at intervals of a day or a few days is necessary, more rapid regeneration is highly desirable, if not essential.
In food and biotechnology applications, it is further frequently desirable that the membrane material be recognized as nontoxic, and preferably on the list of substances generally recognized as safe (GRAS) or cleared for food use by the Food and Drug Administration. In this way, possible contamination of product by membrane material becomes of no concern.
A class of membrane-forming additives of interest in this context comprises saccharinic gums such as alginates, xanthates, pectins, carrageenans, guar, carboxymethyl cellulose, and scleroglucans. Many commercially available forms of these have been cleared for food use. In general, they would be expected to readily form membranes having good ultrafiltration properties, and, in some cases, membranes able to filter dissolved salt to a considerable extent. Examples with alginates and xanthates can be found in a U.S. Environmental Protection Agency report: J. T. McKinnon, EPA-600/2-79-209 (1979).
U.S. Pat. No. 4,851,120 teaches use of polysaccharides and their derivatives as composite membranes, but not as formed-in-place membranes, for separation of water from organics. It teaches use of polyvalent cationic materials to render the membrane less water soluble.
These gum membranes can be easily stripped by exposure to clean-in-place (CIP) solutions, such as hypochlorite or alkaline peroxide, typically used for cleaning and disinfecting at daily intervals in food-processing systems. They can generally be formed in less than thirty minutes, only a small increment to normal cleaning time.
Being GRAS listed and having the attributes described above, the gum formed-in-place membranes would appear to be promising candidates for applications such as pressing or clarifying juices with an "Ultrapress", such as taught in U.S. Pat. No. 4,716,044, as well as for many other food processes. But, in our evaluations of gum formed-in-place membranes, some difficulties have become apparent.
In membranes formed at low pressure [pressures up to about 50 pounds per square inch gauge (psig)], separation performance is erratic since it depends on factors other than pore size of the membrane and the size of material being separated. For example, rejection of bovine serum albumin (BSA) by a sodium alginate membrane formed in place on sintered stainless steel tubes with filter aids present having a pore size of about 0.05 to 0.15 micrometers was found to be dependent on pH and excess salt concentration. At a pH near the isoelectric point of BSA or when charges were shielded by the addition of excess salt, large species (MW about 69,000), which are removed when not near the isoelectric point and in the absence of excess salt, permeated the membrane. Separations achieved with sodium alginate membranes formed at low pressure therefore appear to be largely dependent on coulombic effects.
By forming the membrane at high pressure (greater than 50 psig, preferably greater than 150 psig) such as taught in the previously cited McKinnon report, better separation based on size considerations results. But these membranes tend to be unstable, particularly at elevated temperatures (greater than about 45.degree. C.) that may be desired for various reasons such as increasing flux or operating under Pasteurization conditions. By unstable, it is meant that the gum membranes are too quickly stripped from the substrate by process material to be economically and practically attractive. Even at ambient temperatures, unstable gums have been found to be displaced from formed-in-place membranes when contacted with feeds containing coarse particles. Thus, they are particularly unattractive in applications in which longer membrane lifetimes are desirable.